Beginning in 1953, Spent Nuclear Fuel (SNF) was reprocessed by the Department of Energy to recover highly enriched uranium and other nuclear related products. Processing operations involved multiple cycles of solvent extraction to recover uranium-235 and other defense-related materials from SNF. The end of the Cold War also ended the program to reprocess SNF, with the last reprocessing cycle ending in 1994. These reprocessing activities, as well as other ancillary facility activities and operations, generated millions of gallons of liquid radioactive wastes, which were stored in underground storage tanks.
To mitigate the dangers associated with leakage of these storage tanks, a fluidized bed calcination process was put in operation in the early 1960's to convert the liquid tank waste into a small, granular solid calcine generally having consistency similar to laundry detergent. The calcination process produced a safer product for storage while reducing the volume of stored waste by an average factor of seven. Approximately 8 million gallons (30,300 m3) of liquid tank waste were converted to 4,400 m3 of calcine, which is now being stored while awaiting future disposition.
The disposition of this stored calcined waste is driven by the waste form itself. The waste form determines how well the waste is locked up (chemical durability), as well as the waste loading efficiency, i.e., a higher efficiency requires fewer containers, which reduces disposal cost. The use of glass-ceramic waste forms for problematic wastes such as the calcines, which are difficult to vitrify, offers significant performance improvements and efficiency savings, principally via higher waste loadings. Integral to the design of the waste form is the selection of the appropriate process technology used to treat the calcine.
A key consideration is to select a flexible process that does not constrain the waste form chemistry. Constraints imposed by the consolidation technology on the waste form chemistry will result in a reduction in waste loading efficiency and process flexibility. For instance, Joule-heated melters (JHM) not only have a restricted maximum operating temperature but also require the glass to have specific electrical resistivity and viscosity characteristics. Similar considerations apply to cold-crucible melters. Therefore, the glass cannot be designed solely to suit the waste stream. Additional components need to be added to ensure the glass chemistry is such that it can be melted at the melter operating temperature and poured safely into a canister. These constraints significantly reduce the maximum achievable waste loading efficiency and/or process flexibility and therefore substantially increase the number of waste canisters required.
The Inventors have discovered that by using a novel HIP technology, significant performance enhancements can be realized. These relate to higher waste loadings, enhanced process flexibility, reduced off-gas emissions, competitive production rates and reduction in secondary wastes, while readily complying with the required waste form acceptance criteria outlined by the DOE.